I am also getting undelivered invoices - its nothing to do with wrong e mail address or a temporary problem with the clients e mail - its that QB e mail servers are blacklisted - you have a number apparently and some are blacklisted due to the amount of spam sent.

My IT software guy says one has been released and another later today. 

You guys need to urgently look into this as  we have hundreds of invoices to send out and trhis will not only add a huge amouint of time but affect our cashflow so please GET IT FIXED NOW.